The present invention generally pertains to transponders and more particularly to circuitry for dynamically adjusting the threshold level of a transponder to detect low power interrogation signals while not responding excessively to noise as a result of temperature variation, aging of components or other environmental conditions.
Transponders must be designed to respond to a wide range of input interrogation levels. At low interrogation power levels, the transponder's threshold must be adjusted to detect interrogation signals while rejecting noise. If a transponder responds to noise instead of interrogation signals, errors are generated in the transponder system. As the error rate increases, transponder performance decreases.
As mentioned above, modern transponders require response to low power interrogation signals. If transponder thresholds are set sufficiently high to insure that they will not respond to noise, these low power interrogation signals may be ignored as well. A high threshold setting will have the affect of reducing the false alarm rate, that is responding to noise instead of interrogation signals, but will decrease the sensitivity of the transponder. Thereby, low threshold settings may substantially raise the false alarm rate for modern transponders since these transponders must respond to low power level interrogation signals. These high threshold settings also reduce the sensitivity of the transponder.
To maintain transponder sensitivity at low threshold settings, the transponder circuitry must be very stable. This invention will minimize variations in the transponder's performance. This stability requirement considerably increases the cost of a transponder.
Accordingly, it is an object of the present invention to provide an adaptive threshold control circuit which reduces the cost and increases the reliability of a transponder while maintaining high sensitivity to low power interrogation signals and disregarding noise.